Fixing apparatus preventing leakage of electric current from inner surface of fixing roller

ABSTRACT

A fixing apparatus prevents the offset of toner onto a fixing roller, by forming a surface mold release layer made of pure fluororesin material on a core member of the fixing roller and by applying a bias voltage to the fixing roller.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a fixing apparatus used with an image forming system such as a laser beam priner, copying machine and the like.

[0003] 2. Related Background Art

[0004] An example of a conventional fixing apparatus is schematically shown in FIG. 1.

[0005] A fixing roller 1 is constituted by coating an insulative layer 3 made of fluororesin, silicone resin and the like on a rigid metallic roller, and a pressure roller 2 is pressed against the fixing roller 1. By passing a recording medium on which a toner image formed by the electrophotographic technique is born between the fixing roller 1 and the pressure roller 2, the toner image is fixed onto the recording medium. Further, in this case, in the apparatus shown in FIG. 1, a given DC voltage is applied between the fixing roller 1 and the pressure roller 2 so that the fixing roller contacting the toner image has the same polarity as the toner image.

[0006] In this way, an electrostatic repelling force is generated between the fixing roller and the toner image, thereby preventing the occurrence of a so-called “offset phenomenon” that the toner is adhered to the fixing roller.

[0007] However, in the above-mentioned conventional fixing apparatus, depending upon the surface condition of the fixing roller and the material of the insulative layer, the adequate insulation feature could not be obtained, with the result that the offset phenomenon could not be prevented completely.

SUMMARY OF THE INVENTION

[0008] The present invention aims to eliminate the above-mentioned conventional drawback, and has an object of the present invention to provide a fixing apparatus wherein the offset phenomenon is not generated between a fixing roller and toner.

[0009] Another object of the present invention is to provide a fixing apparatus wherein a transfer blank can be prevented.

[0010] A further object of the present invention is to provide a fixing apparatus which can stabilize a potential on the recording medium.

[0011] A still further object of the present invention is to provide a fixing apparatus which can suppress the generation of noise due to the leak of the bias of a fixing roller.

[0012] Other objects of the present invention will be apparent from following description referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is an elevational sectional view of a conventional fixing apparatus;

[0014]FIG. 2 is a partial sectional view of a fixing roller showing pinholes formed in a surface mold releasing agent layer on the fixing roller;

[0015]FIG. 3 is an elevational sectional view of a fixing apparatus according to a first embodiment of the present invention;

[0016]FIG. 4 is an elevational sectional view of an image forming system incorporating the fixing apparatus according to the present invention;

[0017]FIG. 5 is an elevational sectional view of a fixing apparatus according to a second embodiment of the present invention;

[0018]FIG. 6 is a partial sectional view of a fixing roller wherein a surface mold releasing agent layer is formed as a tube-like configuration;

[0019]FIGS. 7A and 7B are perspective views showing molding processes for forming the tube-like fixing roller;

[0020]FIG. 8 is a partial sectional view of a fixing roller according to a third embodiment of the present invention;

[0021]FIG. 9 is a partial sectional view of a fixing roller according to a fourth embodiment of the present invention;

[0022]FIG. 10 is a partial sectional view of a fixing roller according to a fifth embodiment of the present invention;

[0023]FIG. 11 is a graph showing the relation between a potential on a surface of the fixing roller and the number of sheets passed through the fixing roller;

[0024]FIG. 12 is a graph showing the relation between the potential on the surface of the fixing roller and the time;

[0025]FIG. 13 is a sectional view of a fixing roller according to a sixth embodiment of the present invention;

[0026]FIG. 14 is a graph showing the relation between a potential on a surface of the fixing roller and the number of sheets passed through the fixing roller, regarding two different pressure rollers;

[0027]FIG. 15 is a graph showing the relation between the offset toner gravity and the number of sheets passed through the fixing roller;

[0028]FIG. 16 is a sectional view of a fixing roller according to a seventh embodiment of the present invention;

[0029]FIG. 17 is a sectional view of a fixing roller according to a eighth embodiment of the present invention;

[0030]FIG. 18 is an elevational side view of a fixing roller and a pressure roller, according to a ninth embodiment of the present invention;

[0031]FIG. 19 is a graph showing the relation between the potential on the surface of the fixing roller and the number of sheets passed through the fixing roller, in the ninth embodiment; and

[0032]FIGS. 20 and 21 are elevational sectional side views of a fixing roller and a pressure roller, according to a tenth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] A fixing apparatus according to a first embodiment of the present invention is shown in FIG. 3.

[0034] Since the operating process of the fixing apparatus is the same as that of the aforementioned conventional fixing apparatus, the explanation thereof will be omitted.

[0035] A pure fluororesin layer 18 (having the volume resistance of 10¹⁴ Ω·cm or more) is coated on a conductive core member 16 via a primer layer 17. The fluororesin may be PFA, PTFE, FEP or the like. A pressure roller 20 acting as a back-up roller is urged against a fixing roller 19 constructed as mentioned above. A bias having the same polarity as that of toner is applied to the fixing roller 19 from an electric power source 21.

[0036] As mentioned above, by using the pure fluororesin as a surface mold releasing agent layer of the fixing roller, the surface of the fixing roller is sufficiently insulated, with the result that a sufficient repelling force is generated between the fixing roller 19 and the toner and an attractive force is generated between the pressure roller and the toner, whereby the toner is adequately fixed onto a recording medium, thus preventing an offset phenomenon of the toner regarding the fixing roller.

[0037] However, in the above-mentioned first embodiment, although the problem regarding the offset of the toner was eliminated, the following new problem occurred.

[0038] For example, when the bias voltage of −1.5 kv is applied to the fixing roller 19 constituted by coating PFA resin having a thickness of 25 μm on an aluminium core member 16 coated by the primer layer 17, the current of 5-400 μA will flow as the recording medium is passed through the fixing apparatus. Generally, although the pure PFA resin is insulative material, if a single coating layer is used, as shown in FIG. 2, a great number of imperfect portions (pinholes) H are created in the tissue of the surface separating agent, with the result that the surface mold releasing agent layer does not sometimes act as the insulative layer. In such a case, the current leaks through the recording medium, leading to arise a problem that a transfer mechanism of an image forming system is badly influenced.

[0039] Now, a second embodiment of the present invention will be explained.

[0040]FIG. 4 is an elevational sectional view of an image forming system such as, for example, a laser beam printer to which the present invention is applied. A photosensitive member 23 uniformly charged by a charging means 22 is exposed by an exposure means 24 on the basis of image information to form an electrostatic latent image. The latent image formed on the photosensitive member is changed to a toner image by a developing means 25, and the toner image is transferred onto the recording medium by means of a transfer means 26. The recording medium bearing the non-fixed toner image is fed to a fixing means 27, where the toner image is fixed onto the recording medium by heat and pressure, thereafter the recording medium is ejected out of the image forming system. In this arrangement, a maximum length of the recording medium is greater than a distance between the transfer means and the fixing means.

[0041] The fixing apparatus according to the second embodiment of the present invention used with such image forming system will now be explained.

[0042] First of all, the fixing apparatus according to the second embodiment will be described with reference to FIGS. 5 to 7. Incidentally, the same elements as those shown in FIG. 1 are designated by the same reference numerals, and the explanation thereof will be omitted.

[0043] In this second embodiment, a fixing roller 1 as a rotary member was constituted by coating a core member (conductive core member) 3 made of aluminium, steinless steel or the like with a PFA (pure fluororesin) tube 28. According to this embodiment, as shown in FIG. 6, the PFA tube 28 acting as the surface mold releasing agent layer coated as an outermost layer on the conductive core member does not generate any pinhole, thus providing the excellent insulation feature. The PFA tube 28 in this embodiment has the volume resistance of 10¹⁴ Ω·cm or more (i.e., surface resistance of 10¹² Ω or more).

[0044] A molding process for the fixing roller 1 is schematically shown in FIGS. 7A and 7B. In FIG. 7A, a tube-like thin PFA film 28 previously manufactured in discrete process can easily be formed as a uniform film because it can be formed from the PFA resin solely. After the core member 3 coated by the primer layer is surrounded by the thin PFA film 28 so formed, as shown in FIG. 7B, by applying heat, the thin PFA film is contracted or shrinked, so that the thin film is adhered to the core member 3 through the primer layer.

[0045] The feature of the PFA layer so formed is that it does not include any pinholes H as shown in FIG. 2. Thus, in the illustrated embodiment using the fixing roller having the aforementioned PFA tube, even when the high voltage is applied to the core member, the current does not leak through the recording medium P, and accordingly, if a power source (voltage applying means) 15 having the smaller current capacity is used, it is not feared that the applied voltage is not decreased, thus suppressing the offset phenomenon stably. Further, since it is also not feared that the current leaks to temperature-sensitive elements disposed around the fixing roller, the excellent advantage can be obtained in view of the security and/or the protection of electrical elements. In addition, since the transfer mechanism does not badly influenced upon the delicate leak current, a good image can be obtained.

[0046] Next, a test result will be described.

[0047] The following Table 1 shows the valuation of the leak current, transfer blank and electrostatic offset in comparison between a case where the recording medium is passed through the conventional fixing apparatus using the fixing roller having the PFA coating and a case where the recording medium is passed through the fixing apparatus using the fixing roller having the PFA tube as the illustrated embodiment of the present invention. The valuation was effected both when the power source having the sufficient capacity to apply the core member to a constant voltage of −1.5 kv was used and when the fixing roller was earthed. A diameter of the fixing roller was 30 mm and a thickness of the PFA layer was 25 μm in both cases. TABLE 1 Measuring Leak environ- PFA current Transfer blank Offset ment layer μA −1.5 kv earth −1.5 kv earth 15° C. coating 5 ◯ ◯ Δ X  5% tube 0 ◯ ◯ ◯ X 25° C. coating 10 Δ ◯ Δ X 50% tube 0 ◯ ◯ ◯ X 30° C. coating 400 X Δ ◯ ◯ 80% tube 3 ◯ ◯ ◯ ◯

[0048] As apparent from the Table 1, when the voltage was not applied to the fixing roller, the transfer blank-did not occur both in the cases of the PFA coating and the PFA tube, but, under the measuring environment of low temperature and low humidity, the electrostatic offset noticeably occurred both in the cases of the PFA coating and the PFA tube. The electrostatic offset can be suppressed by applying the voltage (for example, −1.5 kv) having the same polarity as that of the toner to the core member of the fixing roller. However, in the case where the PFA coating was used on the fixing roller, under the measuring environment of high temperature and high humidity, the current leaked to the transfer mechanism, thus generating the transfer blank due to the pinholes formed in the surface mold releasing agent layer consisting of the PFA coating. To the contrary, when the PFA tube was used as the PFA layer as in the illustrated embodiment, even when the voltage was applied to the core member, it was found that both the electrostatic offset and the transfer blank did not occur in all of the measuring environments.

[0049] Next, a third embodiment of the present invention will be explained with reference to FIG. 8. Incidentally, the same elements as the aforementioned conventional case (FIG. 1) and embodiments of the present invention are designated by the same reference numerals, and the explanation thereof will be omitted.

[0050] In this third embodiment, the difference between this embodiment and the first embodiment is that the coating layer for the fixing roller consists of a modified PFA coating layer. That is to say, as shown in FIG. 8, in the process for manufacturing the fixing roller, by compressing a PFA layer 13 to obtain its high density tissue while pressurizing the layer by means of a rigid roller R and then by fusing the compressed layer with heat, it is possible to obtain a modified PFA layer 29 without any pinholes H. The fixing roller having a surface mold releasing agent layer so formed can also attain the same technical advantage as that of the second embodiment for the reason that it can suppress the transfer blank and the offset.

[0051] Further, the surface mold releasing agent layer according to this third embodiment has also advantages that it can be adhered to the core member more strongly than the PFA tube and that it has the excellent endurance.

[0052] Next, a fourth embodiment of the present invention will be explained with reference to FIG. 9.

[0053] In this fourth embodiment, a surface mold releasing agent layer for the fixing roller comprises a plurality of PFA coating layers. According to this embodiment, since an inner PFA coating layer 13A is coated by an outer PFA coating layer 13B so that any pinhole passing through the whole surface mold releasing agent layer (constituted by the plural PFA coating layers), the same technical advantage as that of the second embodiment can be obtained.

[0054] Since the surface mold releasing agent layer (PFA layers) according to this fourth embodiment can have a thickness greatly more than that of the second embodiment, it is possible to greatly increase the resistance between the recording medium and the core member. Accordingly, this embodiment is particularly effective when the fixing roller is used under the high humidity environment where the resistance of the recording medium is noticeably reduced.

[0055] Next, a fifth Embodiment of the present invention will be explained with reference to FIG. 10.

[0056] In this fifth embodiment, a primer layer 30 adhering the PFA coating layer 13 to the core member 3 of the fixing roller has an insulation feature. According to this embodiment, it is possible to maintain the insulation ability of the fixing roller, regardless of the present of pinholes H in the PFA coating layer 13. Thus, it is possible to make the PFA layer (which is expensive) thinner, thereby reducing the cost of the fixing roller.

[0057] As mentioned above, by using the pure fluororesin as the surface mold releasing agent layer of the fixing roller to provide the insulation feature, even when the voltage is applied to the core member of the fixing roller in order to prevent the electrostatic offset, since there is no current leaked from the core member to the recording medium, it is possible not only to maintain the stable offset suppressing voltage but also to prevent the interference with the transfer mechanism, thus eliminating the poor transfer and the like.

[0058] Although the present invention can prevent the transfer blank and the offset in the manners as mentioned above, the present invention provides a fixing apparatus which can also solve the following another problem.

[0059] The charges on a surface of the surface mold releasing agent layer of the fixing roller are increased as the insulation feature of the surface mold releasing agent layer is increased. Consequently, the charges are gradually accummulated on the surface of the layer, with the result that it is feared that the electric field outside of the fixing roller is greatly influenced upon the accummulated charges. The electric field generated by the charging tends to weaken the electric field generated by the voltage applied to the core member for preventing the offset. It was found that the effect of the charging sometimes cancelled the effect of the voltage applied to the core member of the fixing roller, thus generating the offset phenomenon frequently. Accordingly, the above contermeasures may not be sufficient.

[0060] The state of such charging can be ascertained by measuring the surface potential of the fixing roller. When the voltage of −500V is applied to the fixing roller coated by the PFA tube as the mold releasing agent layer for preventing the offset regarding the negatively-charged toner and a number of sheets (recording media) are passed through the fixing roller, the change in the surface potential of the fixing roller will be as shown in FIG. 11. Further, FIG. 12 shows the change in the surface potential of the fixing roller regarding the time period during which one sheet is being passed through the fixing roller, where t₀ indicates a time when a reading end of the recording medium (sheet) has just entered into a nip between the fixing roller and the pressure roller, and t₁ indicates a time when the recording medium has just left the nip.

[0061] As apparent from FIG. 11, before the recording medium is passed through the fixing roller, the voltage of −500V applied to the core member of the fixing roller is maintained as it is, as the surface potential; whereas, after 100 sheets have been passed through the fixing roller, the voltage is decreased to −250V. This is the reason why, while the sheets are passed through the fixing roller repeatedly, the positive charges are accummulated on the surface of the fixing roller by the charging, with the result that the electric field generated by the charges cancels the electric field generated by the potential of the core member. In such a case, the effect of the potential of the core member acting as the offset suppressing means will be almost not effective, and it was ascertained that, after 100 sheets had been passed through the fixing roller, the offset phenomenon was noticeably occurred.

[0062] On the other hand, referring to FIG. 12, it is found that the charging is not generated by the friction between the sheet and the fixing roller but mainly generated by the peeling of the sheet from the fixing roller. Within the time duration from the time t₀ to the time t₁, the surface potential of the fixing roller is almost not changed in spite of the fact that the friction is generated between the recording medium and the fixing roller; whereas, the great surface potential is generated at the time t₁ when the recording medium is peeled from the fixing roller. Such increase in the surface potential generated at the peeling of the recording medium (peel charging) is caused since there is the great potential difference between the potential of the core member of the fixing roller and the surface potential of the recording medium. However, because the purpose for applying the bias voltage to the core member is to urge the toner against the recording medium by the electric field created by such potential difference, the aforementioned peel charging cannot be eliminated theoretically.

[0063] Thus, a more effective fixing apparatus is provided by the following embodiment of the present invention.

[0064]FIG. 13 is an elevational sectional view of a fixing apparatus according to a sixth embodiment of the present invention. In this sixth embodiment, a fixing roller 31 is formed by coating a core member 32 made of aluminium, stainless steel and the like with the pure PFA resin tube 33 as shown in the second embodiment. The coating layer may be formed by performing the multi-coating treatment using fluororesin material or by using material having very high insulation feature and endurance as an adhesive for the fluororesin. The coating layer acting as the insulative layer has preferably the volume resistance of 10¹⁴ Ω·cm or more and surface resistance of 10¹² Ωor more, and the insulative pressure endurance is preferably 1 kV or more.

[0065] The fixing roller 31 used in this embodiment having the PFA tube of the thickness of 50 μm acting as the surface mold releasing agent layer was found that it had the actual resistance of 10¹⁴ Ω or more (measured upon the application of 500 V by using the high resistance measuring device Model 4329A manufactured by Yokokawa Hyulet Packard Co.) and the insulative pressure endurance of 2 kV. The reason for obtaining such high insulative feature and high pressure endurance is that, when the PFA resin is formed in the tube-like shape, the pinholes as shown in FIG. 2 are eliminated from the tissue of the PFA resin tube, thus maintaining the high insulative feature inherent to the PFA resin itself.

[0066] On the other hand, conductive silicone rubber 35 is used as elastic material constituting a surface layer of the pressure roller 34 acting as the back-up roller, and this elastic material is earthed. The pressure roller 34 formed by such conductive elastic material is not charged even if it slidingly contacts with the recording medium, and, accordingly, does not influence the electrostatic repelling force upon the toner, thus improving the offset suppressing effect. It is preferable that the electrical resistance of the conductive elastic material 35 having such characteristic is 10⁵ Ω·cm or less (volume resistance) and becomes 10¹¹ Ω or less (actual resistance) between the surface (10 cm²) of the pressure roller and the core member of the pressure roller (upon application of 500 V). Further, in the arrangement wherein a portion of the recording medium enters into the fixing apparatus while being transfer-charged by the transfer means, i.e., when a length of the recording medium is longer than a distance between the transfer means and the fixing means, if the pressure roller has an extremely low resistance, under the high humidity environment, the current leaks through the recording medium unlimitedly, thus arising the possibility of the poor transfer and the like. Accordingly, it is preferable that the conductive elastic material having the volume resistance of 10²-10⁵ Ω·cm to provide the actual resistance (of the pressure roller) of 10³-10¹¹ Ω; that is to say, it is desirable that the volume resistance value at least near the surface of the back-up roller is smaller than that of the surface mold releasing agent layer (the pressure roller having such construction is referred to as “conductive pressure roller” hereinafter).

[0067] When the voltage having the same polarity as that of the toner is applied to the core member 32 of the fixing roller to suppress the offset phenomenon, by combining the conductive pressure roller with the fixing roller, it is possible to stabilize the potential of the recording medium and to suppress the surface potential of the fixing roller at the minimum, with the result that the offset can be prevented effectively even when a number of sheets are passed through the fixing rollers continuously.

[0068] Next, a test result will be described.

[0069] When the PFA resin layer having the thickness of 300 μm was used as the surface mold releasing agent layer of the fixing roller and the voltage of −500V was applied to the core member of the fixing roller as the offset suppressing bias, the surface potentials of the fixing roller generated when a number of sheets were passed through the fixing roller were measured and compared between a case where the conductive material was used as the elastic layer of the pressure roller and was earthed and a case where the insulative material was used as the elastic layer of the pressure roller. The conductive elastic material used here was selected to have a relatively high resistance value such as the volume resistance of the order of 10⁵ Ω·cm in order to prevent the occurrence of the transfer blank because the fixing apparatus was not sufficiently spaced apart from the transfer mechanism in the arrangement used in this test. Incidentally, in this case, the actual resistance of the elastic material was about 10⁹ Ω.

[0070] As apparent from FIG. 14 showing the test result, when the insulative pressure roller was used, the fixing roller was charged to decrease the surface potential thereof by 400 V or about after 1000 sheets had been passed through the fixing roller; to the contrary, when the conductive pressure roller was used, the surface potential of the fixing roller was merely decreased by about 100 V after 1000 sheets had been passed through the fixing roller.

[0071]FIG. 15 shows the relation between the offset toner gravity and the number of the passed sheets measured in the above-mentioned cases. Incidentally, in this case, the toner was charged negatively. When the insulative pressure roller was used, the amount of the offset toner was increased acceleratively as the number of the passed sheets was increased; to the contrary, when the conductive pressure roller was used, it was found that the amount of the offset toner could be suppressed at the minimum.

[0072] Next, a seventh embodiment of the present invention will be explained.

[0073]FIG. 16 is an elevational sectional view of a fixing apparatus according to the seventh embodiment of the present invention. In this seventh embodiment, the conductive elastic material 35 of the pressure roller 34 is earthed via a rectifier element 36. The rectifier element 36 is connected in a direction that the charges having the polarity opposite to that of the toner can be shifted from the earth side to the elastic material. As a result, the pressure roller is biased to have the polarity opposite to that of the toner, thus providing the effect for suppressing the offset (self-bias effect). Incidentally, the voltage applying means connected to the pressure roller is not limited to the rectifier element, but may comprise a power source, varister, diode or the like. A maximum value of the potential given to the core member of the pressure roller by the self-bias effect in equal to the inverse pressure endurance value of the rectifier element 36. Thus, if the rectifier element having the excessively high inverse pressure endurance is selected, when charged up, not only the charging of the fixing roller is enhanced, but also the charges are discharged to the core member of the fixing roller and other environmental members, thus damaging such members or causing the jamming. Therefore, it is preferable to select the rectifier element so that the maximum potential difference between the core member of the pressure roller and the core member of the fixing roller becomes 3 kV or less.

[0074] Depending on the recent tendency for the compactness of the system, in an arrangement wherein a feeding path of the recording medium from the transfer mechanism to the fixing apparatuses short, if the resistance value of the conductive elastic material in the sixth embodiment is extremely low, the transfer current flow out through the conductive pressure roller to cause the inconvenience such as the transfer blank and the like; however, in such a case, the sixth embodiment of the present invention provides an advantage that the leak of the transfer current is prevented by the rectifier element 36. Accordingly, with the arrangement as mentioned in the sixth embodiment, the conductive elastic material having lower resistance value can also be used.

[0075] Next, a eighth embodiment of the present invention will be explained.

[0076]FIG. 17 is an elevational sectional view of a fixing apparatus according to the eighth embodiment of the present invention. In this eighth embodiment, the surface mold releasing ability of the pressure roller 34 is further improved by providing a conductive PFA tube 37 acting as a surface separating agent layer on the conductive elastic material 35 of the pressure roller. The conductive PFA tube 37 generally has the volume resistance of the order of 10⁷-10¹⁰ Ω·cm greater than that of the conductive silicone rubber. A thickness of the conductive PFA tube is a few tenth μm or less (preferably, 60 μm or less), and, by combining this tube with the conductive elastic material, it is possible to prevent the charging of the pressure roller sufficiently. By providing the surface mold releasing agent layer on the pressure roller, it is possible to prevent the adhesion of the toner onto the surface of the pressure roller considerably in comparison with the pressure roller having the single silicone rubber layer, thus avoiding the adhesion of the recording medium onto the pressure roller and/or the smearing of the back surface of the recording medium.

[0077] In this eighth embodiment, while the conductive PFA tube was used as the surface mold releasing agent layer, the ordinary fluororesin tube or coating may be used as the surface mold releasing agent layer to achieve the same technical effect, so long as the actual resistance value thereof is reduced by making the layer thinner. Further, by using a conductive primer material as a primer layer for such fluororesin surface mold releasing agent layer, it is possible to further reduce the resistance value.

[0078] When the flow-out of the toner holding charges from the recording medium due to the conductivity of the pressure roller becomes an issue, a rectifier element 36 as shown in the seventh embodiment may be incorporated.

[0079] However, with the arrangements as shown in the sixth to eighth embodiments, if the insulation between the fixing roller and the pressure roller is insufficient, it is feared that the voltage cannot be applied to the fixing roller due to the leak of the bias, or there arises the discharge between the fixing roller and the pressure roller to generate the noise, thereby badly influencing upon the control of the system itself and/or the operation of the surrounding equipments.

[0080] A further embodiment of the present invention which can solve such problem will be described hereinbelow.

[0081]FIG. 18 shows a ninth embodiment of the present invention. Since the construction of this embodiment is similar to that of the sixth embodiment, the explanation of the same elements as those of the sixth embodiment will be omitted.

[0082] As apparent from FIG. 18 showing the axial positional relation between the fixing roller and the pressure roller, both ends A of the fixing roller 31 are positioned outwardly of both ends B of the low resistance layer of the pressure roller 34, and a distance between the ends {overscore (AB)} is so selected, regarding the voltage V being applied to the core member of the fixing roller 31, to meet the following relation:

{overscore (AB)}≧V/300+α.

[0083] That is to say, it is so designed that, in the axial direction of the fixing roller, the both ends of the surface separating agent layer of the fixing roller exceed radially outwardly from the both ends of the back-up roller.

[0084] The value {overscore (AB)} was derived from the fact that, if the insulative pressure endurance of the normal gap is 1 KV/mm, the voltage causing the dielectric breakdown is lower and has a lower limit of about 300 V/mm. Further, the value α is a tolerance value generated in the assembling of the apparatus (also in consideration of the sliding discrepancy between the fixing roller and the pressure roller), and is normally 1 mm or less.

[0085] Next, a test result will be described.

[0086] When the PFA resin tube having the thickness of 50 μm was coated on the fixing roller as the surface coating material for the fixing roller and the voltage of −600 V was applied to the core member of the fixing roller as the offset suppressing bias, the surface potentials of the fixing roller generated when a number of sheets were passed through the fixing roller were measured and compared between a case where the conductive material was used as the elastic layer of the pressure roller and was earthed and a case where the insulative material was used as the elastic layer of the pressure roller. The conductive elastic material used here was selected to have a relatively high resistance value such as the volume resistance of the order of 10⁵ Ω·cm in order to prevent the occurrence of the transfer blank because the fixing apparatus was not sufficiently spaced apart from the transfer mechanism in the arrangement used in this test. Incidentally, in this case, the actual resistance of the elastic material was about 10⁹ Ω.

[0087] With this arrangement, the distance between the end of the low resistance layer of the pressure roller 34 and the end of the insulative layer of the fixing roller 31 was 3 mm. It was found that, since the tolerance of the sliding discrepancy of the fixing roller and the like is 1 mm at the most and V/300 equals to 2 mm, the bias voltage applied to the core member of the fixing roller 31 was not discharged to the low resistance layer of the pressure roller 34, so long as the above distance was selected to have a value of 3 mm.

[0088] On the other hand, when the distance between the end of the low resistance layer of the pressure roller 34 and the end of the insulative layer of the fixing roller 31 was 1.5 mm, it was found that the voltage was leaked between the fixing roller and the pressure roller to generate the noise in the surrounding equipments, thus causing the erroneous operation of such equipments.

[0089]FIG. 19 shows the test result indicating the change in the surface potential of the fixing roller measured in such arrangement. As apparent from FIG. 19, when the insulative pressure roller was used, the fixing roller was charged to decrease the surface potential thereof by 400 V or about after 1000 sheets had been passed through the fixing roller; to the contrary, when the conductive pressure roller was used, the surface potential of the fixing roller was merely decreased by about 100 V after 1000 sheets had been passed through the fixing roller.

[0090] In this way, according to this embodiment, the occurrence of the offset due to the leak of the bias and of the noise could be effectively prevented.

[0091] Next, a tenth embodiment of the present invention will be explained.

[0092] Since an elevational sectional view of a fixing apparatus according to this tenth embodiment is the same as that of the seventh embodiment, the illustration thereof will be omitted. In this tenth embodiment, a distance between the end B of the low resistance layer of the pressure roller and the end A of the insulative layer of the fixing roller is indicated, regarding the voltage V′ applied to the pressure roller and the bias voltage V, as follows:

{overscore (AB)}=( V+V′)/300+α.

[0093] Next, an eleventh embodiment of the present invention will be explained.

[0094] Since an elevational sectional view of a fixing apparatus according to this eleventh embodiment is the same as that of the eighth embodiment, the illustration thereof will be omitted. In this eleventh embodiment, a distance between the end B of the low resistance layer of the pressure roller and the end A of the insulative layer of the fixing roller is indicated as {overscore (AB)}=V/300+α.

[0095] Lastly, a twelveth embodiment of the present invention will be explained.

[0096] In the above-mentioned embodiments, while the fixing roller was shown as a cylindrical shape, it may have stepped end portions formed by a drawing process or press fitting process, as shown in FIG. 20. The insulative layer 33 may be extended up to the end surfaces of the stepped end portions.

[0097] In this way, he length of the insulative layer along the contacting surface can be extended and a gap (space) distance L from the core member of the pressure roller can be longer than the gap distance l shown in FIG. 18, thereby preventing the leak and discharge of the voltage through the gap.

[0098] Further, as shown in FIG. 21, the insulative layer may be costituted by the PFA tube and such layer may be protruded inwardly and outwardly from the stepped end portions in a non-adhered condition. In this case, a distance between the end of the low resistance layer of the pressure roller 34 and the core member of the fixing roller 31 can be substantially 2S, and the insulative distance from the core member of the pressure roller can be lengthened, thus preventing the leak of the voltage more effectively.

[0099] In all of the aforementioned embodiments, while the bias voltage was applied to the core member of the fixing roller, a voltage having the polarity opposite to the toner may be applied to the pressure roller and the core member of the fixing roller may be earthed.

[0100] As mentioned above, according to the present invention, the potential of the recording medium can be stabilized, the charging of the surface of the fixing roller can be reduced, and the sufficient offset suppressing effect can be maintained. Further, it is possible to prevent the occurrence of the offset and the occurrence of the noise due to the leak of the bias of the fixing roller.

[0101] It should be noted that the present invention is not limited to the illustrated embodiments, but various alterations and modifications can be adopted within the scope of the present invention. 

What is claimed is:
 1. A fixing apparatus for fixing a non-fixed image on a bearing medium by pinching and conveying the bearing medium bearing the non-fixed image thereon, comprising: a rotary member having a conductive core member and a surface mold release layer; a back-up member pressed against said rotary member; and a voltage applying means for applying bias voltage to said rotary member; wherein said surface mold release layer is made of pure fluororesin material.
 2. A fixing apparatus according to claim 1 , wherein a volume resistance value of said surface mold release layer is 10¹⁴ Ω·cm or more.
 3. A fixing apparatus according to claim 1 , wherein said back-up member comprises a second rotary member.
 4. A fixing apparatus according to claim 1 , wherein a surface resistance value of said surface mold release layer is 10¹² Ω or more.
 5. A fixing apparatus according to claim 1 , wherein said surface mold release layer has a tube-like shape.
 6. A fixing apparatus according to claim 5 , wherein said fluororesin is PFA.
 7. A fixing apparatus according to claim 5 , wherein said fluororesin is PTFE.
 8. A fixing apparatus according to claim 5 , wherein said fluororesin is FEP.
 9. A fixing apparatus according to claim 1 , wherein said voltage applying means comprises a bias power source.
 10. A fixing apparatus according to claim 1 , wherein said voltage applying means applies a bias having the same polarity as a charging polarity of the non-fixed image to said rotary member.
 11. A fixing apparatus according to claim 1 , wherein said rotary member contacts with the non- fixed image.
 12. A fixing apparatus according to claim 1 , wherein said surface mold release layer comprises a plurality of fluororesin coating layers.
 13. A fixing apparatus according to claim 1 , further including a primer layer for adhering said surface mold release layer to said coductive core member, disposed between said conductive core member and said surface mold release layer.
 14. A fixing apparatus according to claim 13 , wherein said primer layer is insulative.
 15. A fixing apparatus according to claim 5 , wherein the fixing apparatus is used with an image forming system for forming an image on the bearing medium.
 16. A fixing apparatus according to claim 15 , wherein said image forming system comprises an image bearing member, and a transfer means for transferring a non-fixed image formed on said image bearing member onto the bearing medium, and wherein a distance between said transfer means and the fixing apparatus is shorter than a length of the bearing medium having a maximum size which can be pinched and conveyed by the fixing apparatus.
 17. A fixing apparatus for fixing a non-fixed image on a bearing medium by pinching and conveying the bearing medium bearing the non-fixed image, comprising: a rotary member having a conductive core member and a surface mold release layer; a back-up member pressed against said rotary member; and a voltage applying means for applying a bias voltage to said rotary member; wherein a volume resistance value near at least a surface of said back-up member is smaller than a volume resistance value of said surface mold release layer.
 18. A fixing apparatus according to claim 17 , wherein the volume resistance value of said surface mold release layer is 10¹⁴ Ω·cm or more.
 19. A fixing apparatus according to claim 17 , wherein the volume resistance value of said back-up member is 10²-10⁵ Ω·cm.
 20. A fixing apparatus according to claim 17 , wherein a surface resistance value of said surface mold release layer is 10¹² Ω or more.
 21. A fixing apparatus according to claim 17 , wherein said back-up member comprises a second rotary member.
 22. A fixing apparatus according to claim 17 , wherein said surface mold release layer is made of pure fluororesin material.
 23. A fixing apparatus according to claim 20 , wherein said surface mold release layer has a tube-like shape.
 24. A fixing apparatus according to claim 23 , wherein said fluororesin is PFA.
 25. A fixing apparatus according to claim 23 , wherein said fluororesin is PTFE.
 26. A fixing apparatus according to claim 23 , wherein said fluororesin is FEP.
 27. A fixing apparatus according to claim 17 , wherein said voltage applying means comprises a bias power source.
 28. A fixing apparatus according to claim 17 , wherein said voltage applying means applies a bias having the same polarity as a charging polarity of the non-fixed image to said rotary member.
 29. A fixing apparatus according to claim 17 , wherein said rotary member contacts with the non- fixed image.
 30. A fixing apparatus according to claim 17 , wherein said surface mold release layer comprises a plurality of fluororesin coating layers.
 31. A fixing apparatus according to claim 17 , further including a primer layer for adhering said surface mold release layer to said conductive core member, disposed between said conductive core member and said surface mold release layer.
 32. A fixing apparatus according to claim 31 , wherein said primer layer is insulative.
 33. A fixing apparatus according to claim 17 , wherein said backup member comprises elastic material.
 34. A fixing apparatus according to claim 28 , wherein said backup member is earthed.
 35. A fixing apparatus according to claim 28 , further including a second voltage applying means for applying a bias voltage to said back-up member.
 36. A fixing apparatus according to claim 19 , further including a conductive tube layer disposed outwardly of said back-up member.
 37. A fixing apparatus according to claim 36 , wherein said conductive tube layer is made of fluororesin material.
 38. A fixing apparatus according to claim 36 , wherein a volume resistance value of said conductive tube layer is 10⁷-10¹⁰ Ω·cm.
 39. A fixing apparatus according to claim 36 , wherein a thickness of said conductive tube layer is 60 μm or less.
 40. A fixing apparatus according to claim 38 , further including a conductive primer layer disposed between said back-up member and said conductive tube layer.
 41. A fixing apparatus for fixing a non-fixed image on a bearing medium by pinching and conveying the bearing medium bearing the non-fixed image, comprising: a rotary member having a conductive core member and a surface mold release layer; a back-up member pressed against said rotary member; and a voltage applying means for applying a bias voltage to said rotary member; wherein a volume resistance value of at least a surface of said back-up member is smaller than a volume resistance value of said surface mold release layer, and wherein both ends of said surface mold release layer exceed outwardly over both ends of said back-up member.
 42. A fixing apparatus according to claim 41 , wherein-the volume resistance value of said surface mold release layer is 10¹⁴ Ω·cm or more.
 43. A fixing apparatus according to claim 41 , wherein the volume resistance value of said back-up member is 10²-10⁵ Ω·cm.
 44. A fixing apparatus according to claim 41 , wherein a surface resistance value of said surface mold release layer is 10¹² Ω or more.
 45. A fixing apparatus according to claim 41 , wherein said back-up member comprises a second rotary member.
 46. A fixing apparatus according to claim 41 , wherein said surface mold release layer is made of pure fluororesin material.
 47. A fixing apparatus according to claim 46 , wherein said surface mold release layer has a tube-like shape.
 48. A fixing apparatus according to claim 47 , wherein said fluororesin is PFA.
 49. A fixing apparatus according to claim 47 , wherein said fluororesin is PTFE.
 50. A fixing apparatus according to claim 47 , wherein said fluororesin is FEP.
 51. A fixing apparatus according to claim 41 , wherein said voltage applying means comprises a bias power source.
 52. A fixing apparatus according to claim 41 , wherein said voltage applying means applies a bias having the same polarity as a charging polarity of the non-fixed image to said rotary member.
 53. A fixing apparatus according to claim 41 , wherein said rotary member contacts with the non-fixed image.
 54. A fixing apparatus according to claim 41 , wherein said surface mold release layer comprises a plurality of fluororesin coating layers.
 55. A fixing apparatus according to claim 41 , further including a primer layer for adhering said surface mold release layer to said conductive core member, disposed between said conductive core member and said surface mold release layer.
 56. A fixing apparatus according to claim 55 , wherein said primer layer is insulative.
 57. A fixing apparatus according to claim 41 , wherein said backup member comprises elastic material.
 58. A fixing apparatus according to claim 52 , wherein said backup member is earthed.
 59. A fixing apparatus according to claim 52 , further including a second voltage applying means for applying a bias voltage to said back-up member.
 60. A fixing apparatus according to claim 43 , further including a conductive tube layer disposed outwardly of said back-up member.
 61. A fixing apparatus according to claim 60 , wherein said conductive tube layer is made of fluororesin material.
 62. A fixing apparatus according to claim 60 , wherein a volume resistance value of said conductive tube layer is 10⁷-10¹⁰ Ω·cm.
 63. A fixing apparatus according to claim 60 , wherein a thickness of said conductive tube layer is 60 μm or less.
 64. A fixing apparatus according to claim 61 , further including a conductive primer layer disposed between said back-up member and said conductive tube layer.
 65. A fixing apparatus according to claim 52 , wherein when the voltage applied to said rotary member is V [V], a difference {overscore (A)} B [mm] in distance between the end of said surface mold release layer and the corresponding end of said back-up member is indicated as the following equation: {overscore (AB)}=V/300 [mm].
 66. A fixing apparatus according to claim 65 , wherein said distance {overscore (AB)} is V/300+α[mm] or more.
 67. A fixing apparatus according to claim 59 , wherein, when the voltage applied to said rotary member is V [V] and an inverse pressure endurance of said second voltage applying means is V′ [V], a difference {overscore (AB)} [mm] in distance between the end of said surface mold release layer and the corresponding end of said-back-up member is indicated as the following equation: {overscore (AB)}=( V+V′)/300+α[mm].
 68. A fixing apparatus for fixing a non-fixed image on a bearing medium by pinching and conveying the bearing medium bearing the non-fixed image, thereon, comprising: a rotary member having a conductive core member and a surface mold release layer; a back-up member pressed against said rotary member; and a voltage applying means for applying a bias voltage to said rotary member; wherein a volume resistance value of said surface mold release layer is 10¹⁴ Ω·cm or more. 